1. Field of the Invention
The present invention is for large aperture light control devices. In particular, this invention is suitable for charge coupled device (hereafter CCD) cameras. Specifically, the present invention provides high sensitivity control for the imager section of a CCD seeker in a weapons system.
2. Description of the Prior Art
Previously, vidicon camera systems were inappropriate for small lightweight applications such as missile system seekers. In general, they are too big and clumsy for use in such systems. They are further limited by their slow response time. The single advantage of vidicon cameras over CCD camera systems is the wide-range sensitivity control of a vidicon camera. The use of charge coupled devices for camera systems has been subject to over-saturation or image blooming. Image blooming refers to the situation when pixels of a charge coupled device in a given area are saturated. Incoming photons in effect spill over into neighboring pixels causing the saturated image to bloom and grow, encompassing a larger and larger area.
To expand the dynamic range of CCD camera systems without use of mechanical irises, the automatic light control, ALC, device was created. The automatic light control device permits the aperture area to remain constant and provides uniform extinction over the aperture area as light intensity increases. Aperture control with irises is not suitable for CCD devices. As the aperture becomes smaller, diffraction effects become increasingly important and eventually limit operation of a device with an automatic iris. Thus, the automatic iris imposes an inherent limit on how far down light control can be reduced before other factors prevent operability. The inherent sensitivity of CCD devices requires a great deal of aperture reduction to control a significant dynamic range. Thus, while a CCD device has relatively small size and weight due to its being a voltage-controlled device, it has significant dynamic range limitations and needs external assistance to be used.
The invention "Absorption Path Controlled Filter" by Theodore B. Bailey permitted CCD devices to be used. The basic feature of the Absorption Path Controlled Filter is its use of an optical absorbing fluid to fill the role of the iris. Basically, the increased depth of an optical working fluid increases the absorption through the aperture area. A fixed optic element and a movable optical element are used with a working fluid reservoir. This reservoir automatically fills the space between the optical elements as the movable element moves away from the fixed element. Previous automatic light control devices have been mechanically driven. This required sliding O-ring seals, automatic volume change adjustment, bearings, high wear parts, no inherent fluid damping, and is especially sensitive to vibration. These systems have backlash or play in the system. All these drawbacks need to be overcome for an automatic light control device to provide inherent reliability and dependability needed in hostile environments. In particular, the nose cone of a missile undergoing massive g-force acceleration is considered a hostile environment for a mechanical or hydraulic device.